text: #if !defined(__MQTT_PAL_H__)
  #define __MQTT_PAL_H__
  
  /*
  MIT License
  
  Copyright(c) 2018 Liam Bindle
  
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files(the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions :
  
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
  
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
  */
  
  #if defined(__cplusplus)
  extern "C" {
  #endif
  
  /**
   * @file
   * @brief Includes/supports the types/calls required by the MQTT-C client.
   *
   * @note This is the \em only file included in mqtt.h, and mqtt.c. It is therefore
   *       responsible for including/supporting all the required types and calls.
   *
   * @defgroup pal Platform abstraction layer
   * @brief Documentation of the types and calls required to port MQTT-C to a new platform.
   *
   * mqtt_pal.h is the \em only header file included in mqtt.c. Therefore, to port MQTT-C to a
   * new platform the following types, functions, constants, and macros must be defined in
   * mqtt_pal.h:
   *  - Types:
   *      - \c size_t, \c ssize_t
   *      - \c uint8_t, \c uint16_t, \c uint32_t
   *      - \c va_list
   *      - \c mqtt_pal_time_t : return type of \c MQTT_PAL_TIME()
   *      - \c mqtt_pal_mutex_t : type of the argument that is passed to \c MQTT_PAL_MUTEX_LOCK and
   *        \c MQTT_PAL_MUTEX_RELEASE
   *  - Functions:
   *      - \c memcpy, \c strlen
   *      - \c va_start, \c va_arg, \c va_end
   *  - Constants:
   *      - \c INT_MIN
   *
   * Additionally, three macro's are required:
   *  - \c MQTT_PAL_HTONS(s) : host-to-network endian conversion for uint16_t.
   *  - \c MQTT_PAL_NTOHS(s) : network-to-host endian conversion for uint16_t.
   *  - \c MQTT_PAL_TIME()   : returns [type: \c mqtt_pal_time_t] current time in seconds.
   *  - \c MQTT_PAL_MUTEX_LOCK(mtx_pointer) : macro that locks the mutex pointed to by \c mtx_pointer.
   *  - \c MQTT_PAL_MUTEX_RELEASE(mtx_pointer) : macro that unlocks the mutex pointed to by
   *    \c mtx_pointer.
   *
   * Lastly, \ref mqtt_pal_sendall and \ref mqtt_pal_recvall, must be implemented in mqtt_pal.c
   * for sending and receiving data using the platforms socket calls.
   */
  
  
  /* UNIX-like platform support */
  #if defined(__unix__) || defined(__APPLE__) || defined(__NuttX__)
      #include <limits.h>
      #include <string.h>
      #include <stdarg.h>
      #include <time.h>
      #include <arpa/inet.h>
      #include <pthread.h>
  
      #define MQTT_PAL_HTONS(s) htons(s)
      #define MQTT_PAL_NTOHS(s) ntohs(s)
  
      #define MQTT_PAL_TIME() time(NULL)
  
      typedef time_t mqtt_pal_time_t;
      typedef pthread_mutex_t mqtt_pal_mutex_t;
  
      #define MQTT_PAL_MUTEX_INIT(mtx_ptr) pthread_mutex_init(mtx_ptr, NULL)
      #define MQTT_PAL_MUTEX_LOCK(mtx_ptr) pthread_mutex_lock(mtx_ptr)
      #define MQTT_PAL_MUTEX_UNLOCK(mtx_ptr) pthread_mutex_unlock(mtx_ptr)
  
      #if !defined(MQTT_USE_CUSTOM_SOCKET_HANDLE)
          #if defined(MQTT_USE_MBEDTLS)
              struct mbedtls_ssl_context;
              typedef struct mbedtls_ssl_context *mqtt_pal_socket_handle;
          #elif defined(MQTT_USE_WOLFSSL)
              #include <wolfssl/ssl.h>
              typedef WOLFSSL* mqtt_pal_socket_handle;
          #elif defined(MQTT_USE_BIO)
              #include <openssl/bio.h>
              typedef BIO* mqtt_pal_socket_handle;
          #elif defined(MQTT_USE_BEARSSL)
              #include <bearssl.h>
  
              typedef struct _bearssl_context {
                  br_ssl_client_context sc;
                  br_x509_minimal_context xc;
                  br_sslio_context ioc;
                  size_t ta_count;
                  br_x509_trust_anchor *anchOut;
                  int fd;
                  int (*low_read)(void *read_context, unsigned char *buf, size_t len);
                  int (*low_write)(void *write_context, const unsigned char *buf, size_t len);
              } bearssl_context;
  
              typedef bearssl_context* mqtt_pal_socket_handle;
          #else
              typedef int mqtt_pal_socket_handle;
          #endif
      #endif
  #elif defined(_MSC_VER)
      #include <limits.h>
      #include <winsock2.h>
      #include <windows.h>
      #include <time.h>
      #include <stdint.h>
  
      typedef SSIZE_T ssize_t;
      #define MQTT_PAL_HTONS(s) htons(s)
      #define MQTT_PAL_NTOHS(s) ntohs(s)
  
      #define MQTT_PAL_TIME() time(NULL)
  
      typedef time_t mqtt_pal_time_t;
      typedef CRITICAL_SECTION mqtt_pal_mutex_t;
  
      #define MQTT_PAL_MUTEX_INIT(mtx_ptr) InitializeCriticalSection(mtx_ptr)
      #define MQTT_PAL_MUTEX_LOCK(mtx_ptr) EnterCriticalSection(mtx_ptr)
      #define MQTT_PAL_MUTEX_UNLOCK(mtx_ptr) LeaveCriticalSection(mtx_ptr)
  
  
      #if !defined(MQTT_USE_CUSTOM_SOCKET_HANDLE)
          #if defined(MQTT_USE_BIO)
              #include <openssl/bio.h>
              typedef BIO* mqtt_pal_socket_handle;
          #else
              typedef SOCKET mqtt_pal_socket_handle;
          #endif
      #endif
  
  #endif
  
  /**
   * @brief Sends all the bytes in a buffer.
   * @ingroup pal
   *
   * @param[in] fd The file-descriptor (or handle) of the socket.
   * @param[in] buf A pointer to the first byte in the buffer to send.
   * @param[in] len The number of bytes to send (starting at \p buf).
   * @param[in] flags Flags which are passed to the underlying socket.
   *
   * @returns The number of bytes sent if successful, an \ref MQTTErrors otherwise.
   */
  ssize_t mqtt_pal_sendall(mqtt_pal_socket_handle fd, const void* buf, size_t len, int flags);
  
  /**
   * @brief Non-blocking receive all the byte available.
   * @ingroup pal
   *
   * @param[in] fd The file-descriptor (or handle) of the socket.
   * @param[in] buf A pointer to the receive buffer.
   * @param[in] bufsz The max number of bytes that can be put into \p buf.
   * @param[in] flags Flags which are passed to the underlying socket.
   *
   * @returns The number of bytes received if successful, an \ref MQTTErrors otherwise.
   */
  ssize_t mqtt_pal_recvall(mqtt_pal_socket_handle fd, void* buf, size_t bufsz, int flags);
  
  #if defined(__cplusplus)
  }
  #endif
  
  
  #endif
  